The primary task of a power transformer is to act as an electric “gear box” and sometimes to create a galvanic isolation, allowing electric energy to flow from one electrical ● system to another. The electrical systems interconnected with a transformer usually have different voltages but always the same frequency. The power transformer, in its simplest form, comprises generally at least two windings, a primary winding and a secondary winding. The transformation ratio is defined by the winding turns in the primary and secondary winding and the connection of the windings, e.g., in “delta” or “Y”-connection.
In the transferring of large powers at high voltages over large distances, the geomagnetic field at changes thereof imposes an often quite large quasi-direct current, (DC) in the power line(-s), so called zero sequence current or GIC, which direct current accompanies the alternating current phase (AC-phase). The phase lines can be regarded as one line over long distances as the distance between each line becomes relatively small, which causes the induction of the DC current, the zero sequence current, to be equal in all phases, when the geomagnetic field is subjected to changes.
The direct current gives rise to unilateral magnetization levels of any transformer in the system, which may cause the core of the transformer to enter magnetic saturation. This leads to the transformer consuming high magnetizing currents, thus being disconnected, normally by means of a protecting system, which releases the transformer from the system. When a transformer is disconnected, released, from the system, this will of course lead to disturbances in the transmission and distribution of electrical energy.
Geomagnetically induced currents (GICs) may, as mentioned above, damage power transformers because of half-cycle saturation of the core and heat developed in iron parts of the transformer. The saturation of the iron core alters the flux paths in the transformers. Parts, such as the tank and press beams, that usually carry only very low flux may be forced to carry much higher force. The increased flux may significantly increase the heat developed in such non-laminated parts of the transformer. The heat dissipation may be so high that the transformer oil starts to boil after a short while.
IEEE Transactions on Magnetics, vol. 35, no. 5, (1999), Transformer Design Considerations for Mitigating Geomanetic Induced Saturation by Viana, W. C. et al discloses the application of an, auxiliary winding used to compensate for GIC. The paper discloses the use of an open delta auxiliary winding which is fed by an adjustable current source. The paper more particularly discusses the placement of the auxiliary winding.
U.S. Pat. No. 1,631,658 discloses a three-phase overhead transmission line with grounded neutral, which line has supply and receiving transformer windings connected into reverse zigzag. By this design fluxes within each transformer column resulting from identical currents in different phases have opposite direction but equal magnitude. The fluxes compensate one another and the resultant total flux is zero. Hereby the transformer cores do not saturate.
Autom. Electr. Power Syst. (China), Apr. 10, 2000, Xue xiangdang et al discloses a geomagnetically induced current compensation at power transformers, wherein FIG. 3 discloses a schematic diagram of compensating GIC by self-excitation, whereby the middle point is connected to ground via actual compensation windings, whereby the transformer becomes self-compensating.
SE patent application S/N 0301893-4 filed Jun. 27, 2003, which corresponds to U.S. patent application Ser. No. 11/3189838 filed Dec. 27, 2005 discloses introduction of a passive compensation system of direct current, zero sequence current, induced by geomagnetic field changes in transforrriers eliminating high magnetization saturation levels, whereby a first impedance (Z1) is arranged from the neutral point to ground in parallel to the compensation winding, which impedance provides a high impedance for low or zero frequencies, and any preferably, a low impedance for higher frequencies
There is hence a strong incentive to prevent direct current to flow through the transformer. As evident from above there are proposals to connect various neutral point devices between the neutral point of a Y-connected transformer winding and earth to reduce or completely eliminate the direct current through transformers. The proposals include: (1) a neutral point resistor, (2) a neutral point capacitor, (3) a DC motor, and (4) elimination of low-impedance neutral point devices only using an overvoltage protective device at the neutral point. One disadvantage with such devices is that the transformer may have graded insulation and the insulation level at the neutral may be too low to withstand the voltage at earth-faults near of the busbar where the transformer is connected. Another disadvantage with such neutral point devices is that they force the direct current to flow through other transformers and makes it necessary to equip also them with neutral point devices.
Geomagnetically Induced Currents flow through transformer windings and create a magnetic field that can saturate the transformer core. This causes the power frequency (50 Hz or 60 Hz) AC magnetic flux to spread out through the windings and structural members of the transformer producing eddy currents that can cause hotspots, which may severely damage the transformer. The magnetising current of the transformer increases significantly during the part of each AC cycle when the magnetic core enters into saturation. The spikes in the magnetising current result in AC waveforms with high harmonic content. These increased harmonics cause incorrect operation of protective relays and may cause disconnection of power lines. The increased reactive power demand accompanied with unwanted operation of protective relays may cause a collapse of power systems.
The geomagnetically induced current is an intermediate variable in the complicated space weather chain starting from the sun and ending in the protection system as indicated in FIG. 1, which is an adaptation of similar charts previously published by Boteler [2] and Pirjola [3].
Aspnes et al. [1 3 have described the complicated process as follows: The Sun is continuously emitting charged particles consisting of protons and electrons into the interplanetary space. This conducting particle flux is called the solar wind. The magnetic field of the Earth could be approximated, as a dipole was it not for the continuous flow of the solar wind. The pressure of the solar wind compresses the magnetic field lines on the sun side of Earth. This distortion of the Earth's magnetic field results in a comet-shaped cavity called the magnetosphere. The protons and electrons, being of opposite charge, are deflected in opposite directions, resulting in an electric current flow. The field aligned currents flow down into the ionosphere. In the lower ionosphere, the protons are slowed by collision with molecules of the atmosphere while the electrons move freely constituting a large current flow called the electrojet. The electrojet is known to be located at least 100 kilometers above the Earth's surface. Electrojet currents of tens of thousands Ampere disturb the magnetic field measured at the surface of the Earth and induce current in the surface of earth.
The induced currents are thus called the geomagnetically induced currents resulting in a time varying earth surface potential. Extended conducting object connected to the earth at several locations tend to shunt the geomagnetically induced current. The objects, like power transmission systems, will, in addition to the fundamental frequency current, carry very low-frequency current. The period of the geomagnetically induced current is usually in the order of minutes and is essentially a direct current in comprising with the fundamental frequency (usually 50 or 60 Hertz).
The current in the power transmission system enters and leaves the power system via earthed neutral points, like transformer neutral. The magnitude of the currents entering and leaving the power system via power transformers may be as high as 300 Ampere. Each winding then carries about ⅓ of the neutral point current and this DC component is very high in comparison with the steady-state fundamental-frequency magnetising current of the transformer. The magnetic material of the core limbs enters into half-cycle saturation. The magnetising current of the transformer becomes very high in comparison with the normal magnetising current. The half-cycle saturated transformer draws a severely distorted current from the power system and distorts the waveform of the voltage on the associated busbar. The general voltage depression, the distorted current and voltage waveforms, and the harmonics may cause incorrect operation of the protection system.